1. Field of the Invention
The present invention relates to a technique for controlling pumps in a multiple pump system; and more particularly relates to a technique for determining whether to stage or de-stage one or more pumps in the multiple pump system.
2. Brief Description of Related Art
Known variable frequency drive systems having multiple pumps will turn-on (stage) or turn-off (de-stage) based on the attainment of specific drive values and their relationship to the current process value. Typical drive values used are speed or torque but can also be values of current or power, if pumps are identical. This methodology utilizes the relationship between the actual process variable and the process variable set point and a designated drive value which must be met before determining if a pump should be staged or de-staged. Each pump in the multiple pump system has its own parameter settings for the stage and de-stage threshold values.
U.S. Pat. No. 8,328,523, entitled “Synchronous Torque Balance in Multiple Pump Systems,” assigned to the assignee of the instant patent application, discloses a method and apparatus for operating and running multiple pumps together in a system using synchronous torque to achieve a desired set point, which is hereby incorporated by reference in its entirety.
By way of example, shortcomings of the above mentioned devices may include the following:
The setup of variable frequency drive systems having multiple pumps as described in above referenced techniques may be both cumbersome and time consuming. It may also be difficult to determine the appropriate de-stage values without trial and error particularly in systems where load balancing (synchronous torque control) is important or where pumps do not have matched performance. This may result in pumps not turning on or off as required. It can also result in cycling whereby a pump frequently turns off and then back on again due to incorrect threshold values. Additionally, this methodology does not address energy efficiency. If the operating number of pumps is not optimum it will result in higher operating cost.
Further, various types of multiple pump controllers such as variable speed drives (VFD), programmable logic controllers (PLC), distributive control systems (DCS) and SCADA systems proliferate in centrifugal pump processes. In these arrangements, multiple pumps operate in a coordinated fashion to satisfy a set point. Typically, these systems will operate with variable speed drives in a synchronous fashion where all pumps operate at the same speed to achieve a desired set point. The objective of synchronous speed control is to balance the flow of all pumps equally by having all pumps run at the same speed. However, in practice this may not necessarily result in balanced flow. Often multiple pumps are not identical and their head curves may not be closely matched. This can result in uneven flow contribution by each pump which can cause excessive wear and tear on the pump. This problem can be addressed by having the multiple pump system operate in synchronous torque control whereby all pumps operate at the same torque but at slightly different speeds to satisfy the set point. By balancing the torque the total load is shared between the operating pumps and flow balance among pumps is greatly improved.
By way of further example, shortcomings of these further mentioned devices may include the following:
While both of these systems have been successfully employed; both arrangements have similar shortcomings. The setup of these systems is cumbersome and time consuming. Quite often de-stage values are determined through trial and error during commissioning of the system. It is essential to have the correct threshold values to stage or de-stage a pump to prevent premature staging and de-staging or cycling whereby pumps are prematurely turning on and off. Extreme cases could result in a decrease of system reliability and unscheduled maintenance of the equipment. The costs associated with unscheduled maintenance include the repair of equipment, interruptions in production and costs associated with environmental cleanup. Incorrect threshold values also increase operating cost over the life of the system.
The typical multiple pump system has parameters for stage values for each pump. The stage values dictate the required speed that the lead pump must be at before multiple pump operation is allowed by the first lag pump. The multiple pump system will automatically start a lag pump when it is unable to maintain the process set point with the current pumps operating providing the speed or torque value is at or above the stage setting.
As system demand decreases, the need for additional pumps will also decrease. Each pump in the multiple pump system has a parameter for a de-stage value. The de-stage parameters identify when pumps are turned off in normal multiple pump control. The primary consideration is to insure that when a lag pump is shutdown the remaining pumps are capable of meeting the process demand. The de-stage setting defines how much of a speed decrease or torque decrease can occur before a pump can be de-staged.
For all the aforementioned reasons, there is a need in the industry for a better way to stage or de-stage one or more pumps in a multiple pump system.